<p>A new method is developed to synthesize Au/Ag alloy nanoparticles (Au/Ag-NPs). The obtained Au/Ag-NPs are further dispersed in polyvinyl alcohol (PVA) solution to construct Au/Ag hydrogel surface enhanced Raman scattering (SERS) chips through a simple Ca²⁺-mediated aggregation of Au/Ag-NPs and a physical crosslinking of PVA. The formed aggregated Au/Ag-NPs (a-Au/Ag-NPs) show a strong localized surface plasmon resonance corresponding to the commonly used 532&#xa0;nm laser. Therefore, under the action of 532&#xa0;nm laser, the abundant nanogaps contained in the a-Au/Ag-NPs turn to high active electromagnetic “hot spots”, providing strong electromagnetic enhancement (EM). Moreover, Au/Ag-NPs provide a moderate Fermi level of -4.72&#xa0;eV, which facilitates its photo-induced charge transfer with many molecules under 532&#xa0;nm laser irradiation, resulting in the chemical enhancement (CM) effect. The synergistic effect of EM and CM leads to ultrahigh SERS activity. Additionally, the unique structure of hydrogel provides excellent signal uniformity and robust anti-interference ability. Thus, the obtained Au/Ag hydrogel SERS chips are ideal SERS platform for the quantitative determination of many molecules. For instance, a sensitive and reliable SERS method is established for the simultaneous detection of methyl parathion and triazophos in fruits and tea.</p> Graphical Abstract <p></p>